Businesses today generate and maintain enormous volumes of data. As a result, storage is no longer an afterthought; instead, it is a strategic investment priority for businesses of all sizes. In fact, businesses are searching for more ways to efficiently manage their expanding volumes of data, and to make such data accessible throughout the enterprise. This trend is propelling the move of storage into the network and, importantly, increasing businesses' attraction to storage service providers. A Storage Service Provider (SSP) is a company that provides data storage space and related management services (e.g., periodic backup and archiving).
Storage Service Providers (SSPs) offer a variety of storage systems. Storage systems are commonly deployed within a storage area network (SAN) or a network attached storage (NAS) environment. In a NAS configuration, a storage server is implemented in the form of an appliance that attaches to a network. NAS systems generally utilize file-based access protocols; therefore, each client may request the services of the storage system by issuing file system protocol messages to the file system over the network.
A SAN is a high-speed network that enables establishment of direct connections between a storage system and its storage devices. With SAN, clients' requests are specified in terms of blocks, rather than files. Conceptually, a SAN may be viewed as an extension to a storage bus that enables access to stored information using block-based access protocols over the “extended bus.” In this context, the extended bus is typically embodied as Fiber Channel or Ethernet media adapted to operate with block access protocols. Thus, a SAN arrangement allows decoupling of storage from the storage system, such as an application server, and placing of that storage on a network.
Today, Fibre Channel is the predominant architecture upon which SAN implementations are built. Fibre Channel is a technology standard that allows data to be transferred from one network device to another at extremely high speeds. However, because dedicated cabling is currently required to implement a Fibre Channel SAN, it cannot run over a public IP-based network.
To form a geographically distributed SAN, a mapping layer can be used, such as to map SCSI over Fibre Channel Protocol (FCP) over IP (FCIP) or to map SCSI over TCP/IP (iSCSI). Internet SCSI (iSCSI) is a data storage networking protocol that transports SCSI commands over the standard TCP/IP networking technology (e.g., the Internet). One difference between FCIP and iSCSI is that iSCSI takes raw SCSI commands and encapsulates the commands into IP packets, whereas FCIP requires that the SCSI commands first be encapsulated into FCP frames before the FCP frames are encapsulated into IP packets. This encapsulation adds additional processing and bandwidth overhead, so FCIP compares poorly to iSCSI as a standalone method of transferring data through an IP-based network.
Compared to a Fibre Channel installation, iSCSI is inexpensive and there are no distance limitations (i.e., Fibre Channel range limit is about 10 km using single-mode fiber). Also, iSCSI can be implemented within any storage server that includes a network connection and a TCP/IP stack; whereas, with Fibre Channel, storage servers require a dedicated SCSI adapter, which is more expensive than an off-the-shelf Ethernet Network Interface Card (NIC).
Because storage management is complex, it is understandable why, as storage concerns have taken precedence for many businesses, SSPs have grown increasingly more attractive as a solution to physically outsource data storage. However, there are presently a number of disadvantages associated with enterprise adoption of a pure IP-based SAN (iSCSI) model. For example, with iSCSI, a public (static) IP address must be assigned to each host that connects to the SSP, and the number of public IP addresses is scarce. As another example, SSPs generally license their services and applications based on the number of hosts connecting to the storage system. Typically, many (if not all) storage servers (i.e., iSCSI hosts) in an enterprise will need to connect to the storage system. Thus, under the current model, businesses are discouraged by escalating licensing costs from adopting a pure IP-based SAN implementation.